Set debugger breakpoint at end of a function without return

Question

I am debugging method f() that has no return in it.

class A(object):      def __init__(self):         self.X = []      def f(self):                     for i in range(10):             self.X.append(i) 

I need to see how this method modifies variable X right after it is called. To do that, I insert a return at the end of the method, and set the breakpoint there:

That way, as soon as the method reaches its return, I can see the value of my variable X.

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This does the job, but I am pretty sure there is a better way. Editing a method or function every time I need to debug it seems silly.

Question:
Is there a different way (e.g. an option in the debugger) to set a breakpoint at the end of a method that does not have a return?

(Note that setting a breakpoint at the function call and using Step Over would not display X when mouseovering, since the function is called from a different module.)

Answer 1

You can add a conditional breakpoint on the last line and set the condition to be something that occurs only in the last iteration.

In this instance the condition is very easy since it's just i == 9, but it may be a lot more complex depending on your loop condition so sometimes adding a statement at the end will be the easier solution.

That screenshot is from IntelliJ IDEA and your screenshot looks like it's from the same IDE, so just right-click the breakpoint to show the dialog and enter your condition.

If you're using some other IDE I'm sure there is capability to make a breakpoint conditional.

Update:

There is no support for breaking at the end of a method in the Python debugger, only at the start of a method:

b(reak) [[filename:]lineno | function[, condition]]

With a lineno argument, set a break there in the current file. With a function argument, set a break at the first executable statement within that function. The line number may be prefixed with a filename and a colon, to specify a breakpoint in another file (probably one that hasn't been loaded yet). The file is searched on sys.path. Note that each breakpoint is assigned a number to which all the other breakpoint commands refer.

If a second argument is present, it is an expression which must evaluate to true before the breakpoint is honored.

Without argument, list all breaks, including for each breakpoint, the number of times that breakpoint has been hit, the current ignore count, and the associated condition if any.

Answer 2

Your IDE is hiding what's under the hood. That is, something like

import pdb 

is prepended to your script and

pdb.set_trace() 

is inserted before the line onto which your placed your breakpoint. From what you say I deduce that PyCharm does not like placing breakpoints on empty lines. However pdb.set_trace() can perfectly be placed at the end of a method.

So you could insert those yourself (or write a macro) and run python -m pdb to start debugging.

(Edit) example

import pdb  class A(object):      def __init__(self):         self.X = []      def f(self):         for i in range(10):             self.X.append(i)         pdb.set_trace()  if __name__ == '__main__':     a = A()     a.f() 

Debug with

$ python -m pdb test.py  > /dev/test.py(1)<module>() ----> 1 import pdb       2        3 class A(object):  ipdb> cont --Return-- > /dev/test.py(11)f()->None -> pdb.set_trace() (Pdb) self.X [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] (Pdb) 

ipdb can be used instead of pdb.